The present invention relates generally to rolled steel bars and more particularly to rolled steel bars for making high strength, high toughness coil and leaf springs, to the methods for producing such springs from the rolled steel bars and to the resulting springs.
A leaf spring typically comprises a plurality of leaf spring leafs assembled together to form a multi-layered spring, but it can comprise only a single leaf. Coil and leaf springs of the type to which the present invention relates are typically used in automobiles or other vehicles for shock resistance.
It is desirable for such springs to be composed of steel having a relatively high hardness (e.g. above Rockwell C (R.sub.c) 52) because the corresponding relatively high tensile strength produces improved resistance to fatigue and to sag on the part of the spring. However, in the past, constraints have been imposed upon the maximum hardness of steels employed for springs because a hardness above R.sub.c 52 could result in premature failure due to poor fracture or notch toughness. Fracture toughness is usually expressed in K.sub.Ic units for a given hardness level. Fracture toughness usually decreases with an increase in hardness.
For example, there is a conventional, commercially available steel, identified as SAE 5160, which contains 0.56-0.64 wt. % carbon. A modification of SAE 5160, identified as SAE 9259, includes 0.75 wt. % silicon. When the SAE 9259 steel was heat treated to a hardness of R.sub.c 54, the fracture toughness was less than 27 MPa.m.sup.1/2. The SAE 9259 steel could be treated to produce a fracture toughness greater than 27 MPa.m.sup.1/2, but this toughness could be obtained only by heat treating to a hardness less than R.sub.c 52. More particularly, the SAE 9259 had a fracture toughness of 36.5 MPa.m.sup.1/2 for a hardness of R.sub.c 48 and 33.0 MPa/m.sup.1/2 for a hardness of R.sub.c 51; but the SAE 9259 steel heat treated to a hardness of R.sub.c 54 had a fracture toughness of only 26.7 MPa.m.sup.1/2.
It would be desirable to produce a spring composed of steel having a hardness of at least R.sub.c 52 together with a fracture toughness substantially greater than 27 MPa.m.sup.1/2.